I'm still running Snow Leopard on my late-2009 mini due to some of the so-so comments here about ML. However, over the past few weeks it's become almost impossible to do anything with files in the Finder. I can't trace this to any specific event, though there have been a few security updates recently. The beachball spins almost constantly. I can't even scroll through a file folder without it locking up for several minutes with the CPU pegging at 100% for the Finder.

Here are some of the things I've tried after doing a few web searches - still no luck:

1. Repaired Permissions multiple times using Disk Utility, ONYX, and CCleaner. Interestingly, each of these indicates that the permissions are repaired but the next time I run the utility (even immediately after competing) it shows the same permissions as being incorrect and repaired again. Seems odd.

2. deleted com.apple.mrt.plist from the Library

3. deleted com.apple.finder.plist

4. deleted com.apple.loginwindow.plist

I think I deleted a few other preference files as well but nothing has helped. All of my apps work fine once they're launched. It's just the Finder that freezes and pegs the CPU. I'm getting tired of waiting, watching the beachball, and listening to the fan scream from the CPU heat.

Here's a screenshot of the Activity Monitor with Finder pinned at 100%.

I had Preview, Activity Monitor, and Firefox running at the time and attempted a simple scroll in a Finder window. The system freezes and the beachball spins and spins. Hope this helps with the diagnosis.

I would suggest against upgrading to ML until your issue is resolved.
It's the worst thing you could do at this point. A fresh install, yes, but an upgrade, no.

First

Try starting in Safe Mode.

Try creating another user account and see if it is acting the same way with Finder.

If you can make a bootable clone of your hard drive, before you proceed, you can always go back to your current state.

In the future this is why I make bootable clones, when I know my system is operating fine and before I do OS updates. Your troubleshooting could have been resolved in seconds.

Ie system messed up.... Boot off clone... System is good ... Clone external back to internal and you are back in business.

Download SuperDuper (free) or CarbonCopyCloner

If you try the things I mentioned and you are still having issues I can talk with my Apple technicians at work and run your scenario by them. We've been an Apple authorized service center for 34 years. Likely they've seen this before.

Thanks Bandit Bill,
I think it's somewhat back to normal with the exceptions of the dock preferences and other things that I deleted (easily fixed with a bit of work).

I booted it in Safe Mode and all sorts of odd things started happening when I later logged back in regular mode. The finder would open, flash, and disappear within a couple of seconds. I tried the other user account that I have on the mini and it worked fine. So I started focusing on my account and the things that it's running.

It appears that the menubar utility for iDriveSync was the culprit for making the Finder flash and disappear. I'm not sure that it was the cause for making Finder peg at 100% since that seemed to go away after the Safe Mode boot.

So now the mini is functioning close to normal and I'll take another backup before things get worse again.

I own a purchased copy of SuperDuper and use it frequently, but not as often as I should. I need to get better about doing a backup before adding any software or system updates. I also use Time Machine (occasionally) for file backups but wouldn't count on it for a major catastrophe.

Anyway, I greatly appreciate the advice. I think it's running OK again but time will tell.

Before I forget, now that the mini seems to be healthy again, are there any other maintenance things I should do? As I mentioned, I occasionally fix the permissions and clean out debris with ONYX and CCleaner. Is there anything else that I should or could be doing?

I'm guessing that you're still not recommending an upgrade (or as some have said, a downgrade) to Mountain Lion.

At some point in the near future a new mini or possibly an iMac will be in my future but I'd like to get a little more mileage out of this one, if possible.

Once I'm comfortable that the system is running fine, I clone my HD again

repeat with each update.

I also run Time Machine for ongoing backups, but I never rely on Time Machine. I just look at it as a way to access accidentally deleted files and an extra back-up copy just incase something by chance goes wrong with the clone and internal HD.

With this routine, I've never had an issue, since I purchased my first mini in 2005.

I'm going to keep SL on my 2007 mini for the rest of it's time. I upgraded it to Lion and then went back to SL. Your machine is a bit newer, so it will handle ML fairly well, given it has 8GB RAM.

You can always set up a dual boot system being your machine is fully capable of doing so. If you find you like ML then you can move on.

Part of my choice to keep my mini with SL is due to the fact that I have an iMac that I run ML on.

If I had a 2009 mini with 8GB of RAM like yourself, and the knowledge that you have, I'd suggest the dual boot system. You have the hardware and the knowhow to do both. ML is only $20 so $ really isn't an issue. It also sounds like you like to learn, so ML as a learning experience will be of value to you.

I have ML on all my machines now. The only downsides for me was the "Save As" issue and removing the web sharing option from sys prefs. The only Mac program I used heavily was Preview so I had to just deal with it until they added "save as" back with the option key. I also used screen sharing a lot and I couldn't get it work between versions. Other than that, all my various applescripts, shell scripts, automator actions, web sites, etc all work. I am sure there were some other issues but they so insignificant that I can't even remember them anymore.

I only have 4GB in my mini, it was Mooblie who posted the screenshot of the 8GB machine. Since things are running normally again I think I'll stick with Snow Leopard for the time being, or until I start seeing more software that requires Mountain Lion as a prereq.

The dual-boot idea does sound intriguing and I may give it a shot if I have a slow weekend in my future.

Again, I appreciate the helpful advice from the forum on fixing my issue, on the possibility of upgrading, and on better preventative steps to keep things running!

I only have 4GB in my mini, it was Mooblie who posted the screenshot of the 8GB machine. Since things are running normally again I think I'll stick with Snow Leopard for the time being, or until I start seeing more software that requires Mountain Lion as a prereq.

The dual-boot idea does sound intriguing and I may give it a shot if I have a slow weekend in my future.

Again, I appreciate the helpful advice from the forum on fixing my issue, on the possibility of upgrading, and on better preventative steps to keep things running!

Yes, I looked at Mooblie's screen shot thinking it was yours oops

I'd stick with SL if you are sticking with 4GB of RAM. Your computer will run SL just fine with 4GB. 8GB is a welcomed upgrade with ML even if your needs are very basic. I ran my iMac with 4GB in SL, no issues, when I went to ML I noticed page outs until I went to 8 GB (this is with light use running 2 user accounts).

This started happening to my Core 2 Duo 2.4 ghz as well when I installed iDriveSync. The finder would start to hang within a few seconds of opening a new window, and simple scrolling or click-n-dragging caused the whole thing to seize. Bummer, because they give you 10 GB storage free. I guess I'll just leave it off normally and turn it on periodically to sync.